allow specifying migrations and schema paths with root-relative path strings

This removes the need for the special `default_*` methods.  It also
is a more developer-friendly way of setting custom paths for these
settings.

If this was not in place, the developer would have to remember to
pass in a correct, absolute path string.
ensure some limited error info is written on all cli cmds

There were some cases where a cli cmd would fail (ie on create or
drop) and no error info was shown.  This ensures a limited (ie no
backtraces) set of error info is always shown.